Setup project to be used as a gem.

- Add gemspec file for building mlbschdules gem.
- Remove call to command line in mlbschedules.rb file.
- Update readme to explain building and installing gem locally.
Add mlbschedules executable for use with gem.

- Add mlbschedules file that calls the command line static method.
- Add executable to gemspec.
- Fix usage string in OptionParser.